The present invention relates to the field of optical devices and is particularly concerned with a polarization selective phase shifting, splitting and combining optical device.
Optical fibers are becoming an increasingly popular data transmission medium since they are believed to be ideally suited to the requirements of high-speed communication networks. Some of the desirable characteristics of optical fiber networks are that they transmit signals over relatively long distances with relatively low signal losses and at relatively high rates. Fiber bandwidth is thus a critical characteristic of optical fiber networks since fiber bandwidth is directly related to the information carrying capacity of a fiber and limits the maximum rate at which information can be transmitted.
The search for higher bandwidth has resulted in the deployment of improved optical transmission systems. These high-speed networks have brought new challenges to the optical components industry. Optical phenomena, which have had negligible effects on system performance in the past, are now of utmost importance. Polarization is a common factor in a number of such phenomena that must be characterized if these high-bandwidth systems are to meet, and perhaps exceed, their potential.
As is well known in the art, the polarization of light is determined by the time course of the direction of the electric-field vector. For monochromatic light, the three components of the electric-field vector vary sinusoidally with time with amplitudes and phases that are generally different, so that at each position the endpoint of the vector moves in a plane and traces an ellipse. The plane, the orientation and the shape of the ellipse vary with position.
In paraxial optics, however, light propagates along directions that lie within a narrow cone centered about the optical axis. Waves are approximately transverse electromagnetic and the electric-field vector therefore lies approximately in the transverse plane. A polarized light signal can thus be divided between an x axis polarization component and an orthogonal y axis polarization component.
In instances wherein one of the polarization components is zero or wherein both polarization components are in phase, then the light signal is said to be linearly polarized and can be represented by a simple vector that has a given amplitude and a given angle relative to the reference axes. If the two polarization components are of same amplitude but out of phase relative to each other, then the polarization state is said to be circular. If the two polarization components are of different amplitude and phase, the polarization is said to be elliptical.
In modern fiber optic telecommunications, the polarization of the signal is typically used to help direct the signal along the fiber optic network. Network components or devices which function based upon the polarization of the light signal include polarization division multiplexers, polarizers, depolarizers, fiber optic polarization tunable filters, binary polarization switch/modulators, and many other polarization related fiber optic components. All of these devices require fiber optic variable polarization beam splitters and/or combiners that are adapted to either split a light signal into two orthogonal linearly polarized signals or to combine optical signals by reversing their paths in the device.
Polarization beam splitters and/or combiners are not only used as part of other optical components but are also used alone or in combination with other optical devices in a variety of situations. For example, polarization beam combiners may prove to be particularly useful in the context of signal amplification. Although modern fibers have very low losses per unit length, signal amplification is an important element of many optical information networks. Indeed, long fiber spans, for example, cables extending from one city to another, require periodic amplification of the transmitted signal to ensure accurate reception at the receiver.
Erbium doped fiber amplifiers have been developed to satisfy this need for signal amplification. Such amplifiers consist of a length of optical waveguide fiber, typically 5 to 30 meters of fiber, which has been doped with erbium. The quantum mechanical structure of erbium ions in a glass matrix allows for stimulated emission in the approximately 1520 to 1620 nanometer range, which is one of the ranges in which optical waveguide fibers composed of silica exhibit low loss. As a result of such stimulated emission, a weak input signal can achieve more than a hundred fold amplification as it passes through a fiber amplifier.
To achieve such stimulated emission, the erbium ions must be pumped into an excited electronic state. Such pumping can take place in various pump bands. Combining/splitting devices are an integral part of the amplification process being used along with semiconductor laser sources and wavelength multiplexing devices for generating a pumping signal.
One common method of producing a polarization splitter involves the use of a birefringent crystal. The splitter works by taking advantage of the anisotropic structure of this crystal; that is, the crystal does not have the same optical density for the two transverse propagation vectors.
When a randomly polarized signal is passed through a crystal of this kind the polarization is broken up into two components relative to the optical orientation of the crystal. Both beams will emerge linearly polarized, but with polarization orientations perpendicular to each other.
Only certain types of crystals will exhibit birefringent behavior. Crystals must have hexagonal, tetragonal, or trigonal lattice structures to allow the light to encounter an asymmetric structure. Some common materials with these characteristics are calcite (calcium carbonate), quartz, and tourmaline. There are many ways to make a beam splitter cube from these materials, the most common being slicing a rectangular prism of the material along a diagonal, and cementing it back together in a different orientation.
Some devices have gain widespread acceptance despite their numerous drawbacks. The Glan-Thompson polarizer, for example, includes a block of birefringent material cut into prisms and then cemented together. It reflects one polarization component at the cement interface and transmits the other. The device suffers from requiring a considerable amount of birefringent material, generally calcite, which is scarce and expensive. It is also unable to work with high-powered lasers and ultraviolet light, since the light destroys or clouds cement. Furthermore, this beam splitter, which makes use of the reflected polarization component, suffers from the added disadvantage that polarized beams exit the device at inconvenient angles, for example 45 degrees, when it is often useful that beams are parallel, orthogonal or otherwise oriented.
The Glan-Taylor polarizer, which is similar to the Glan-Thompson polarizer but uses an air space instead of cement to separate polarization components, can work with many light sources but suffers from reflection loss and ghosting caused by the air gap. The Wollaston, Rochon and Senarmont beam splitters, which separates polarization components by transmitting the components through an interface, permit optical contacting for use with most light sources, but produce beams which also exit at inconvenient angles, with one or both polarization components suffering from chromatism and distortion.
The double refracting element that produces parallel-polarized beams of light, achieves small beam separation and limited field. Also, since the beams may pass through a considerable amount of material before achieving useful separation, wavefront distortion can occur in the extraordinary beam due to imperfections in the crystal""s structure. Beam separation can be further limited by the small size and high cost of suitable crystals.
Other types of known polarization beam splitters and combiners make use of semiconductors. These types of devices also suffer from serious drawbacks. For example, an integrated optical polarization beam splitter using LiNbO3 or semiconductor substrates presents the shortcomings of high insertion loss, poor polarization extinction ratios and limited operational spectral bandwidth.
Yet, other types of known polarization beam splitters and combiners make use of so-called polarization-maintaining fibers. As the name implies, these fibers permit light to pass through them while preserving its polarization state. The term is used to distinguish these fibers from conventional so-called single-mode fibers. As is well known, despite the accepted terminology, single-mode fibers actually propagate two modes, one of each polarization.
If the fiber has a perfect circular geometry and symmetry of its index profile, the two polarization modes travel in the same way and behave as a single-mode. Ideally this means that the fiber must have perfect geometry and be completely symmetrical along the optical axis. It must also be homogeneous along the axis. Such fibers are referred to as low birefringence fibers. In such fibers, any polarization launched into the fiber travels relatively undisturbed, with very little distortion along the fiber and the polarization at the output is substantially identical to that at the input.
Most practical single-mode fibers, however, are not perfect, and even if they were, bending them for example when packaging into cables disrupts their perfect characteristics. Consequently, the two polarization modes in standard single-mode fibers typically travel at slightly different speeds and, hence, arrive at different times at the output. This imposes a form of pulse spreading or bandwidth limitation known as Polarization Mode Dispersion (PMD).
In addition to the two polarization modes arriving at different times, polarized light launched into standard single-mode fibers is not preserved. Instead, it quickly becomes arbitrary and unpredictable at the output. This behavior arises because temperature fluctuations and external forces both perturb the fiber""s geometry and index of refraction, thereby affecting the two polarization modes in different ways. This, in turn, leads to instability in the fiber""s PMD and state of polarization.
To overcome the variable polarization, Polarization Maintaining (PM) fibers have been developed. Since it seems realistically futile to attempt manufacturing a perfectly cylindrical fiber immune to external forces, fiber manufacturers make PM fibers by taking the opposite extreme of deliberately destroying the fiber""s circular symmetry. A different geometry or index of refraction on the fiber""s x and y axes produces an optical loss or velocity much higher on one axis than the other. If the loss of one polarization mode is much higher than the other, the fiber is a true single-mode, single-polarization fiber. Such fibers, however, are not commercially available.
More common are so-called birefringent fibers in which the phase velocity of one polarization mode is much higher than the other. The internal stresses in these fibers are greater than those commonly encountered from external forces. Consequently, the propagating light is little affected by the external forces and almost no power exchanges between the two polarization modes. Typically, standard single-mode fibers have low birefringence, and good commercial PM fibers have high birefringence.
Depending on how light is launched into a birefringent fiber, the state of polarization will either stay the same or vary with distance along the fiber. The amount of power coupled into the unwanted mode is called the Extinction Ratio (ER) and can be used as a measure of the polarization holding ability of a PM fiber. It is defined as ER(dB)=xe2x88x9210 log (PU/PE) where PU is the power at the output in the initially unexcited mode and PE is the power at the output in the excited mode.
If linearly polarized light is not launched exclusively onto either axis, some power propagates in both polarization modes and the phase between the two evolves and periodically returns to its original state. The length of fiber required for the two modes to return to the initial state is the fiber""s beat length. Beat length is a direct measure of birefringence. Hence, fibers having short beat lengths are said to have high birefringence.
Birefringent fibers can be manufactured by deliberately making the fiber asymmetrical in shape or in stress. Because it""s difficult to obtain strong birefringence using only shape variation, most commercial PM fibers achieve most of their birefringence by relying on asymmetrical internal stresses to preferentially change the index of refraction along each axis. This is done by using materials that have different expansion coefficients, and by using an asymmetrical geometry to induce asymmetrical stresses.
In accordance with one method of manufacturing, birefringence is achieved by starting with a preform composed of four regions: a geranium-doped core with a silica clad, two aluminum doped stress rods, six silica rods and a silica tube that encapsulates everything.
Drawing the preform into a fiber at low tensions merges the various materials causing the aluminum-doped stress rods to take the shape of two fan blades. The different thermal expansion coefficient of the stress rods causes large internal stresses in the fiber producing its polarization holding properties. In accordance with another manufacturing method the polarization maintaining fiber achieves internal stresses by using a stress cladding whose thickness varies along two perpendicular axes.
During the process, a circular preform is squeezed into a rectangular shape that is then drawn into a rectangular fiber. As the fiber cools, the different thermal contractions induce internal stresses that coincide with the principal axes of the rectangular shape. The fiber""s rectangular shape provides a geometrical reference that can be used for splicing fibers together. Because it""s important when splicing PM fibers to align not only their cores but also their polarization axes, the fiber""s rectangular geometry enables accurate alignment without the need for monitoring with elaborate equipment. A variation on the rotary splice allows the fiber to be either passively or actively aligned.
While polarization maintaining fibers solve the polarization mode dispersion and polarization instability shortcomings of standard single-mode fibers, the higher cost and optical loss of commercially available PM fibers currently precludes their widespread use in outside plant cables. However, PM fibers find usage in short lengths for connecting optical components and in optical sensors where their higher loss is not a limitation. As mentioned previously, they are also used in some optical devices such as certain types of polarizing beam splitters and combiners.
One example of a polarization splitting and combining device using polarization maintaining fibers is disclosed in U.S. Pat. No. 6,175,668 issued Jan. 16, 2001 and naming Nicholas F. Borrelli et al. as inventors. The disclosed device includes a so-called antipodal phase generator for processing an incident polarized light signal. The antipodal phase generator splits the polarized light into mutually perpendicular polarization components. So-called antipodal signals are created when a first polarization component having a given orientation is phase delayed by an odd multiple of xcfx80 radians with respect to a second polarization component having the same orientation but propagating in the other optical path.
The other polarization components of the light signal are both perpendicular relative to the first and second polarization components and propagate in their respective optical paths while remaining in-phase relative to one another. The disclosed splitting and combining device also includes a coupler that creates a destructive interference between the antipodal signals allowing them to be sent to a first output port while the in-phase polarization components are combined and directed to a second output port.
U.S. Pat. No. 6,175,668 also discloses the herein above described polarization splitting and combining device being used as the basis for an isolator/circulator and a polarization controller. Various embodiments of these optical devices are disclosed, some of which using planar coupler technology while others use fiber coupler technology. All of the disclosed embodiments using fiber coupler technology include optical paths made entirely of polarization maintaining fiber. The polarization maintaining fibers are used both in the phase shifting and signal coupling sections of the devices.
Various methods are disclosed in U.S. Pat. No. 6,175,668 for creating phase delays in the various devices using fiber coupler technology. All of the disclosed methods are based on the use of polarization maintaining fibers extending integrally throughout the entire length of the respective optical paths. In accordance with one of the disclosed methods, the phase delay is created by providing a first and a second polarization maintaining fiber, the fibers being characterized as having different core ellipticities so as to create a relative difference in the length of the corresponding first and second optical paths. In accordance with another disclosed method, the phase shift is created by flexures formed in both integral polarization maintaining fibers.
With regards to the coupling sections of the fiber coupler technology devices disclosed in U.S. Pat. No. 6,175,668, again, they are described as being made out of integral segments of polarization maintaining fibers merging integrally with the polarization maintaining fiber segments responsible for phase shifting of the optical signal. The coupling sections of the fiber technology devices are more specifically disclosed as being of the conventional evanescent-type but using polarization maintaining fiber. Segments of the polarization maintaining fiber are disposed within a glass tube which is heated and collapsed around the fiber segments to form an overclad. The heated device is then drawn to reduce the diameter thereof so as to form the evanescent couplers.
Although somewhat useful, the optical devices disclosed in U.S. Pat. No. 6,175,668 suffer from numerous important drawbacks. These drawbacks are, at least in part, related to the use of optical paths made entirely of integral polarization maintaining fibers both at the phase shifting and signal coupling sections of the devices.
The idea of using polarization maintaining fibers instead of conventional fibers in the coupling sections may seem theoretically appealing. However, in practice, it leads to various serious potential drawbacks including loss of bandwidth or wavelength dependency and increase manufacturing costs. As is well known in the art, an evanescent-type coupler such as suggested for use in the device disclosed in U.S. Pat. No. 6,175,668 is one in which optical energy is transferred from one optical fiber to the another by virtue of the electric-field overlap between the two cores of the fibers. Since the evanescent field of an optical fiber is an exponentially decaying field, the cores of the two fibers must be brought into close proximity.
In general, evanescent-wave couplers are usually constructed using one of two methods namely etch and twist or asymmetric polish. In these two fabrication methods, the cladding layers of the fibers are preferentially removed by chemical etching or mechanical polishing techniques. The fibers are then placed in contact with one another and carefully aligned to achieve optical coupling.
U.S. Pat. No. 6,175,668 and other publications sometimes refer to fused bi-conical taper couplers as evanescent-wave couplers although theoretically these coupler are not really of the true evanescent type. The fused bi-conical taper couplers wave couplers are fabricated by heating two optical fibers until they coalesce into a composite wave-guiding structure. While the fibers are being heated, they are slowly stretched and tapered. This causes the light in the fiber to spread out far enough into the composite structure where it can be coupled to the other fiber.
Typically, fused optical fiber couplers are attractive for use in optical fiber communication networks because they have low loss, good temperature stability, mechanical rigidity and ease of manufacture. Indeed, conventional couplers made by the familiar technique of fused tapering, exhibit exceptionally low loss, but the coupling ratio depends on the wavelength of light passing through the coupler. Since the wavelength of optical sources can vary over considerable ranges, and since it often is necessary to use more than one optical wavelength in a system, it is highly desirable to use couplers that exhibit reduced sensitivity of coupling ratio to optical wavelength.
Couplers having a nominally constant coupling ratio over the wavelength band of interest are referred to as wideband couplers, wavelength flattened couplers, wavelength independent couplers, broadband couplers, etc. By using couplers that have a sufficient constancy over a sufficient bandwidth, with acceptable excess loss and sufficient constancy in performance with changes in temperature and stress environment, systems can be provided for working over a design range of wavelengths and environmental conditions. It follows that improvement in the constancy of coupling ratio while keeping excess loss within acceptable limits can lead to important improvement in the performance and lower cost for many optical systems.
One method of achieving relatively wideband couplers is to produce the couplers with fibers having different propagation constants. A difference in propagation constant between the fibers may be acquired through various methods. In one approach identical fibers are processed to have different diameters, for example, by drawing one fiber into a tapered section of reduced diameter relative to the other fiber. The tapered section is then fused with an unprocessed fiber or with a fiber that was tapered more or less than the first fiber.
More specifically, the wide wavelength range optical fiber coupler includes a first optical fiber, a portion of which has undergone a preliminary elongation, and a second conventional optical fiber. To form the optical fiber coupler, the initially elongated region of the first optical fiber and a section of the second optical fiber are aligned side by side and mutually thermally fused to form a fused section. The fused section thus formed is then elongated or drawn out to form a fused-elongated region.
By virtue of the above-mentioned preliminary elongation process, it is possible to create a propagation constant difference between the component first and second optical fibers. By so doing, it is possible to increase the mode coupling between the component first and second optical fibers across the fused-elongated region, and thereby attain a desired coupling ratio. Different propagation constants can also be obtained by etching one or both fibers so that their diameters are different before fusion or by selecting fibers with different V numbers.
The use of polarization maintaining fibers in the coupling regions as suggested in U.S. Pat. No. 6,175,668 would, at the very least, potentially greatly increase the complexity of the herein above disclosed manufacturing process used to make relatively wavelength insensitive couplers. Indeed, if polarization maintaining fibers were to be used, the fibers would potentially need to be aligned relative to each other with respect to the polarization axes.
Furthermore, the alignment between the fibers would potentially need to remain intact during the various manufacturing steps including the actual drawing process in the case of evanescent-type couplers The difficulty associated with maintaining the alignment between the fibers would potentially be compounded by the fact that at least one of the fibers would be pre-stretched.
Hence, the use of polarization maintaining fibers in the coupling regions as suggested in U.S. Pat. No. 6,175,668 is associated with numerous drawbacks. The use of polarization maintaining fibers instead of conventional fibers in the coupling sections may lead to a potential loss of bandwidth since conventional methods for producing relatively wavelength independent couplers are not particularly well suited for use with polarization maintaining fibers. Other coupler manufacturing methods may prove to be not only more complex, with consequent potential increases in manufacturing costs and potential loss of reliability, but also less susceptible of producing a relatively wavelength independent coupler.
The use of polarization maintaining fibers instead of conventional fibers throughout both branches of the interferometer as suggested in U.S. Pat. No. 6,175,668 is also associated with potentially serious drawbacks such as loss of bandwidth and/or increased manufacturing complexity. Indeed, in order to produce interferometer branches providing a phase shift with reduced sensitivity to optical wavelength, it is desirable that the phase shift between the phase shifted optical components be as small as possible. Hence, it is desirable that the phase shift has a value of xcfx80 radians. In order to achieve such a result, it is, in turn, desirable that the birefringence differential between the respective birefringence level of the interferometer branches be as high as possible and the branches as short as possible.
Hence, if the sensitivity to optical wavelength in the region of the interferometer of the devices disclosed in U.S. Pat. No. 6,175,668 is taken into consideration, the length of the branches of the interferometer needs to be rigorously controlled and ideally quite small. This imposes yet another difficult manufacturing constraint in the manufacturing process. This additional constraint in itself may prove to be at least partially insurmountable again leading to potential loss in bandwidth.
In short, although U.S. Pat. No. 6,175,668 discloses combining and splitting devices all including a so-called antipodal phase generator that theoretically address some of the other prior art device drawbacks such as loss of bandwidth and overall manufacturing complexity, in practice the disclosed devices, nevertheless, suffer from some of the same drawbacks. The drawbacks associated with combining and splitting devices disclosed in U.S. Pat. No. 6,175,668 are at least in part related to the use of optical paths made entirely of integral polarization maintaining fibers both at the phase shifting and signal coupling sections of the devices. Accordingly, there exists a need for an improved optical coupling and splitting device.
Advantages of the present invention include that one of the proposed optical devices allows for the splitting of a light signal into two orthogonal linearly polarized signals. The proposed optical device also allows for combining of optical signals by reversing their paths in the device. The proposed optical splitting and combining device is specifically designed so as to reduce the sensitivity of the device to optical wavelength at strategic regions of the device namely both at the coupling and phase shifting regions of the device.
The coupling regions of the proposed device are specifically designed so as to provide the attractive characteristics associated with fused-optical fiber couplers such as low loss, good temperature stability, mechanical rigidity and ease of manufacture. The coupling regions of the proposed device allow for the benefits of these characteristics with the added substantial benefit of being designed so as to be manufacturable through conventional manufacturing processes known to provide an important reduction of sensitivity to bandwidth.
The phase shifting branches of the proposed device are designed so as to provide relatively small phase shift, typically in the range of one xcfx80 radians so as to provide phase shifting branches with reduced sensitivity to optical wavelength. Also, the phase shifting branches of the proposed device are designed so as to provide a relatively small phase shift without imposing difficult manufacturing constraints such as the need for the length of the branches to be quite small and rigorously controlled.
Overall, one of the proposed optical devices is thus specifically designed so as to allow for both signal splitting and combining while providing large bandwidth. The proposed combining and splitting device is also adapted to be inserted into an optical circuitry for providing another optical device, namely an improved optical polarizer offering the same advantages as the proposed optical splitting and combining device when taken in isolation.
In accordance with an embodiment of the invention, there is provided a Mach-Zehnder device for use with an optical signal, the Mach-Zehnder device comprising a device input port and a device output port; a substantially achromatic first coupling region optically connected to the device input port; a substantially achromatic second coupling region optically connected to the device output port; an intermediate region optically connected to both the first and second coupling regions, the intermediate region including a first intermediate branch and a second intermediate branch, the first intermediate branch defining a phase shifting portion, the phase shifting portion defining a phase shifting portion level of birefringence over the length of the phase shifting portion, the phase shifting portion level of birefringence being different then an intermediate branch reference portion level of birefringence prevailing over at least an intermediate branch reference portion part of the remainder of the first intermediate branch. Preferably, the phase shifting portion level of birefringence is different then the level of birefringence of the remainder of the first intermediate branch and the level of birefringence of the second intermediate branch.
Conveniently, the phase shifting portion level of birefringence is higher then the intermediate branch reference portion level of birefringence. Preferably, the phase shifting portion is made out of a phase shifting segment of polarization maintaining fiber. Preferably, the phase shifting segment of polarization maintaining fiber defines a phase shifting segment length, the phase shifting segment length having a value substantially in the range of one half of a beat length.
Conveniently, the first and second coupling regions are made of first and second fibers fused together, the first and second fibers having different propagation constant about the first and second coupling regions so as to define a coupling region propagation constant differential. Preferably, the propagation constant differential results from a difference in diameter between the first and second fibers about the first and second coupling regions.
Preferably, the phase shifting portion defines a phase shifting portion fast polarization axis and the Mach-Zehnder device further includes a polarization orienting means optically connected to the input port for ensuring that when the optical signal reaches the input port, the optical signal is linearly polarized and defines a signal fast polarization axis substantially aligned with the phase shifting portion fast polarization axis. Conveniently, the polarization orienting means includes a polarization orienting segment of polarization maintaining fiber, the polarization orienting segment of polarization maintaining fiber defining a polarization orienting segment fast polarization axis substantially aligned with the phase shifting portion fast polarization axis.
In accordance with one embodiment of the invention, the Mach-Zehnder device comprises a first main fiber, the first main fiber defining a first fiber input segment, the first fiber input segment being optically connected to a first fiber first coupling segment, the first fiber first coupling segment being optically connected to a first intermediate branch, the first intermediate branch being optically connected to a first fiber second coupling segment, the first fiber second coupling segment being optically coupled to a first fiber output segment; a second main fiber, the second main fiber defining a second fiber input segment, the second fiber input segment being optically connected to a second fiber first coupling segment, the second fiber first coupling segment being optically connected to a second intermediate branch, the second intermediate branch being optically connected to a second fiber second coupling segment, the second fiber second coupling segment being optically coupled to a second fiber output segment; the first fiber first coupling segment and the second fiber first coupling segment being optically coupled to one another so as to form a first coupling region; the first fiber second coupling segment and the second fiber second coupling segment being optically coupled to one another so as to form a second coupling region; the first intermediate branch defining a phase shifting portion, the phase shifting portion including a phase shifting segment of polarization maintaining fiber, the phase shifting portion defining a phase shifting portion level of birefringence over the length of the phase shifting portion, the phase shifting portion level of birefringence being different then an intermediate branch reference portion level of birefringence prevailing over at least an intermediate branch reference portion part of the remainder of the first intermediate branch.
The present invention also relates to a method for forming a Mach-Zehnder device using a first and a second main fiber, the Mach-Zehnder device being intended for use with an optical signal splittable into first and second split signals each having respective fast and slow polarization components, the first and second main fibers respectively defining first and second fiber input segments, first and second fiber first coupling segments, first and second fiber intermediate segments, first and second fiber second coupling segments and first and second fiber output segments, the first and second fiber intermediate segments being adapted to respectively transmit first and second split signals, the method comprising the steps of: splicing a phase shifting segment of relatively highly birefringent fiber in the first fiber intermediate segment between the first fiber first and second coupling segments, the phase shifting segment being calibrated so as to create a polarization selective phase shift between fast and slow polarization components of the first split signal; juxtaposing the first fiber first and second coupling segments respectively with the second fiber first and second coupling segments so as to respectively form first and second device coupling regions.
Conveniently, the method further comprises the step of splicing an input segment of relatively highly birefringent fiber to both the first and second fiber input segments, the polarization axes of one of the input segments of relatively highly birefringent fiber being substantially aligned with the polarization axes of the phase shifting segment of relatively highly birefringent fiber and the polarization axes of the other one of the input segments of relatively highly birefringent fiber being substantially perpendicular relative to the polarization axes of the phase shifting segment of relatively highly birefringent fiber.
Preferably, one of the first or second fiber first or second coupling segments is tapered prior to juxtaposition with a corresponding first or second fiber first or second coupling segments so as to create an asymmetry between the juxtaposed segments and allow for the creation of relatively achromatic coupling region.
Conveniently, the method further comprises the step of calibrating the optical properties of the second fiber intermediate segment so that when the second split signal reaches the second coupling region the second split signal is substantially in phase with one of the polarization components of the first split signal.
In accordance with the present invention, there is also provided a phase shifting device for substantially achromatically modifying the phase of a first optical signal relative to the phase of a second optical signal, the first optical signal defining a first signal fast polarization axis and a first signal slow polarization axis, the second optical signal defining a second signal fast polarization axis and a second signal slow polarization axis, the antipodal phase generator comprising: a first optical path having a first birefringence level, the first optical path defining a first path input and a first path output; a second optical path having a second birefringence level substantially similar to the first birefringence level, the second optical path defining a second path input and a second path output; the first optical path being provided with a phase shifting segment made out of a polarization maintaining fiber having a phase shifting birefringence level relatively different then the first and second birefringence levels, the phase shifting segment defining a phase shifting segment fast polarization axis and a phase shifting segment slow polarization axis, the phase shifting segment being calibrated so as to cause a predetermined phase delay between the first signal fast polarization axis and the first signal slow polarization axis so as to defined a delayed and an non-delayed first signal; the second optical path being calibrated so as to propagate the second signal fast polarization axis substantially in phase with the second signal slow polarization axis and with the non-delayed first signal; the difference in birefringence level between the phase shifting birefringence level and the first and second birefringence levels defining a birefringence differential, the birefringence differential being calibrated such that when the first optical signal is propagated in the first optical path and the second optical signal is simultaneously propagated in the second optical path, the birefringence differential creates a polarization selective phase shift between the first optical signal and the second optical signal.
In accordance with the present invention there is still further provided an optical device for transmitting a first optical signal and a second optical signal, the optical device comprising a first optical path, the first optical path defining a first optical path input port and an opposed first optical path output port, the first optical path being provided with a phase shifting segment optically connected between the first optical path input port and the first optical path output port, the phase shifting segment being made out of a polarization maintaining fiber and having a phase shifting birefringence level, the remainder of the first optical path having a reference birefringence level, the phase shifting birefringence level being different then the reference birefringence level; a second optical path, the second optical path defining a second optical path input port and an opposed second optical path output port, the second optical path having a second birefringence level; the difference in birefringence level between the phase shifting birefringence level and the reference birefringence level defining a birefringence differential, the birefringence differential being calibrated such that when the first optical signal is propagated in the first optical path and the second optical signal is simultaneously propagated in the second optical path, the birefringence differential creates a polarization selective phase shift between the first optical signal and the second optical signal.